If the car's not cranking then first check it can turned over by hand so double checking for any mechanical issues
If OK then check that the main battery cables are on tight and the terminals are clean and bright. Do the same for the engine / gearbox earth strap. Then follow the main positive lead back to the engine and down to the starter motor, and check the connection at this end as well.
I'd check that you're getting full power to the glow-plugs, they pull a lot of current so take care when checking but see if the relay is clicking when the ignition is turned on first and also check the fuse if this is OK check to see if the engine earth strap is also OK.If the plugs haven't been changed in the last 20K then they may have to be replaced.